5. Concluding remarks
We have computed a new chromospheric-activity index based on CORALIE high-resolution echelle spectra. After transforming our values into the Mount-Wilson "S" scale, this index gives us the possibility of checking the activity of the stars of the Geneva extra-solar planet search programme in the southern sky. After almost two years, we have values of for more than 400 stars in the CORALIE sample.
We applied our results to the study of the relation between activity and radial-velocity variation to show that a clear trend exists between radial-velocity "jitter" and the chromospheric activity index for the F, G and K dwarfs in our sample. In general, higher activity dwarfs have higher radial velocity "jitter". This relation is nonetheless not the same for all spectral types, K dwarfs being more "stable" in velocity than G or F dwarfs: the radial-velocity scatter increases with decreasing , confirming former results (e.g. Saar et al. 1998).
The obtained relations may be very useful when dealing with extra-solar planet searches using radial-velocity techniques. Depending on the observed radial-velocity rms, we can use the derived results together with the value of of the star to exclude (or not) activity related phenomena as the cause of the observed radial-velocity variation.
The current results are, however, not "precise" enough to permit us to consider the activity diagnostic as satisfactory and able to clearly distinguish activity related phenomena from real planetary candidates. As more data will be gathered we expect to better clarify and precise the observed relations. In the meanwhile, we think that the use of other activity induced radial-velocity indicators, like the shape of line profiles as seen in the bisector analysis (e.g. Queloz et al., in preparation), may be essential.
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000